Because of difficulties in obtaining large quantities of Pneumocystis carinii organisms, alternative approaches need to be investigated for providing antigens and enzymes that can be used in subsequent studies. With this goal in mind, in collaboration with investigators at University of California, San Francisco, a genomic DNA and a cDNA library have been generated using Pneumocystis carinii obtained from rats. This library is currently in the process of being screened by DNA probes as well as by antibodies to try to isolate probes that contain the major surface antigens of Pneumocystis carinii as well as those that contain enzymes that are potential therapeutic targets for treating Pneumocystis carinii pneumonia, such as dihydrofolate reductase or dihydropteroate synthetase. Our collaborators in San Francisco have already isolated and sequenced a ribosomal RNA gene for rat Pneumocystis carinii, and have documented by sequence homology that Pneumocystis carinii is much more closely related to fungi than to protozoa, as had been previously thought. The importance of this work is that it will provide reagents that will be used in a number of subsequent studies. Specifically, recombinant antigens can be used to better understand the epidemiology of, and the immune response to Pneumocystis carinii infection. The ability to produce large quantities of functional enzyme will allow rapid in vitro assays to evaluate the utility of specific classes of drugs to inhibit such enzymes, and thus to screen for potential therapeutic agents that can be used to treat Pneumocystis carinii pneumonia.